With the spark plug at the center of the piston, the flame front only needs to travel the radius of the piston (vs the diameter of the piston when the plug is located over to the side).
With less distance for the flame front to travel, the “yet-to-be-burned” gasses at the end of the piston have less time to rise and detonate (from the rising pressure).
In addition to the previously mentioned hemi advantages, this allows for more optimal spark advance timing without worry for detonation.